1. Field of the Invention
The present invention relates to micro-oscillating elements which include a rotationally displaceable part, such as micromirror elements, acceleration sensors, angular speed sensors, vibration elements and so on.
2. Description of the Related Art
In recent years, efforts have been made for making practical use of elements which have a micro-structure formed by micromachining technology. In the field of optical communications technology for example, micromirror elements which are tiny elements capable of reflecting light are gathering attention.
In the optical communications, optical fibers serve as a medium through which optical signals are passed. When the optical signal passing through a given optical fiber is switched to another optical fiber, so-called optical switching devices are used in general. In order to achieve high quality optical communications, the optical switching device must have such characteristics as high capacity, high speed and high reliability, in switching action. In view of these, micromirror elements manufactured by utilizing micromachining technology are gathering attention as a switching element to be incorporated in the optical switching devices. The micromirror elements enable the switching operation without converting optical signals into electric signals between the optical paths on the input side and the output side of the optical switching device. This is advantageous in achieving the above-mentioned characteristics.
Micromirror elements have mirror surfaces for reflecting light, and are capable of varying the direction of light reflection by pivoting the mirror surfaces. Many micromirror elements which are used in optical devices are electrostatically-driven. The electrostatically-driven micromirror elements are roughly divided into two types: the micromirror element manufactured by so called surface micromachining technology, and the micromirror element manufactured by so called bulk micromachining technology.
In the surface micromachining technology, material thin film is formed on a substrate, into a desired pattern according to component parts to be made. Such a pattern is laminated in sequence to form supports, mirror surfaces, electrodes and other component parts which constitute the element as well as sacrifice layers which will be removed later. On the other hand, in the bulk micromachining technology, etching is made to a material substrate itself, whereby supports, mirror regions and so on are made into desired shapes. Thin-film formation is performed as necessary, to form mirror surfaces and electrodes. Technical details of the bulk micromachining technology are disclosed in the following Patent Documents for example: JP-A-10-190007, JP-A-10-270714, and JP-A-2000-31502.
Micromirror elements must satisfy a technical requirement that the mirror surface which is responsible for reflecting light should be highly flat. However, according to the surface micromachining technology, the finalized mirror surface is thin and susceptible to warp, and it is difficult to achieve a high level of flatness over a large area of mirror surface. On the contrary, according to the bulk micromachining technology, a relatively thick material substrate is etched to form mirror regions, and mirror surfaces are made on this mirror regions. Therefore, it is possible to provide rigidity to a large area of mirror surface, and as a result, it is possible to form mirror surfaces which have sufficiently high optical flatness.
FIG. 30 is a partially unillustrated exploded perspective view of a conventional micromirror element X4 manufactured by the bulk micromachining technology. The micromirror element X4 includes: a mirror support 41 which has an upper surface provided with a mirror surface; a frame 42 (partially unillustrated); and a pair of torsion bars 43 connecting these. The mirror support 41 has a pair of ends formed with a pair of comb-tooth electrodes 41a, 41b. The frame 42 is formed with a pair of inwardly extended comb-tooth electrodes 42a, 42b correspondingly to the comb-tooth electrodes 41a, 41b. The torsion bars 43 provide an axis for oscillating action of the mirror support 41 with respect to the frame 42.
According to the micromirror element X4 which has the structure as described, a set of comb-tooth electrodes which are placed closely to each other for generation of electrostatic force, e.g. the comb-tooth electrode 41a and the comb-tooth electrode 42a, are apart from each other, making an upper and a lower tiers as shown in FIG. 31A when no voltage is applied. When a voltage is applied on the other hand, as shown in FIG. 31B, the comb-tooth electrode 41a is drawn in between the comb-tooth electrode 42a, pivotally displacing the mirror support 41. More specifically, when the comb-tooth electrodes 41a, 42a are supplied with a predetermined voltage and whereby the comb-tooth electrode 41a is positively charged and the comb-tooth electrode 42a is negatively charged, then there is electrostatic attraction developed between the comb-tooth electrodes 41a, 42a, which rocks the mirror support 41 around the axis A4 while twisting the torsion bars 43. By utilizing such oscillating motion of the mirror support 41, it is possible to switch light reflecting directions in which light is reflected by the mirror surface 44.
FIGS. 32A-32D show a manufacturing method for the micromirror element X4. In these figures, views of a section will be given to illustrate a process of forming those components which are shown in FIG. 30, i.e. part of the mirror support 41, the frame 42, the torsion bars 43 and part of a pair of comb-tooth electrodes 41a, 42a. The section represents a section of a material substrate (a wafer) to which the manufacturing processes is performed, and more specifically a section of a single block from which a single micromirror element is formed. The section includes sections of a plurality of component regions, and the sectional views are illustrative sequential depictions.
In the method of manufacturing the micromirror element X4, a material substrate 400 as shown in FIG. 32A is prepared first. The material substrate 400 is a so called SOI (Silicon on Insulator), which has a laminate structure including a silicon layer 401 and a silicon layer 402, and an insulation layer 403 between them. Next, as shown in FIG. 32B, the silicon layer 401 is subjected to anisotropic etching via a predetermined mask to form structural components (mirror support 41, part of the frame 42, the torsion bars 43 and the comb-tooth electrode 41a) on the silicon layer 401. Next, as shown in FIG. 32C, the silicon layer 402 is subjected to anisotropic etching via a predetermined mask to form structural components (part of the frame 42 and the comb-tooth electrode 42a) on the silicon layer 402. Next, as shown in FIG. 32D, the insulation layer 403 is subjected to isotropic etching to remove exposed portions of the insulation layer 403. In this way, formation is made for the mirror support 41, the frame 42, the torsion bars 43 and the comb-tooth electrodes 41a, 42a. The other set of comb-tooth electrodes 41b, 42b are formed in the same way as the comb-tooth electrodes 41a, 42a. 
According to the micromirror element X4 as described above, when rotational displacement of the mirror support 41 is 0°, the comb-tooth electrodes 41a, 42a are spaced from each other in the direction of thickness of the element as shown in FIG. 31A and FIG. 32D. Therefore, when the comb-tooth electrodes 41a, 42a are supplied with a predetermined voltage, electrostatic attraction generated between the comb-tooth electrodes 41a, 42a is not appropriately effective. In addition, the comb-tooth electrodes 41b, 42b are spaced from each other in the direction of thickness of the element, and therefore when the comb-tooth electrodes 41b, 42b are supplied with a predetermined voltage, electrostatic attraction generated between the comb-tooth electrodes 41b, 42b is not appropriately effective. For these reasons, when moving the mirror support 41 from the rotational displacement of 0° by generating electrostatic attraction between the comb-tooth electrodes 41a, 42a or between the comb-tooth electrodes 41b, 42b, it is sometimes impossible to achieve sufficient response necessary for the movement. Likewise, when stopping the mirror support 41 at the rotational displacement of 0°, it is difficult to control the electrostatic attraction highly accurately around the stopping point, which leads to so called residual vibration, i.e. a phenomenon that the mirror support 41 keeps oscillating near the rotational displacement of 0°, making impossible for the mirror support 41 to make an immediate stop. As understood, the micromirror element X4 has a problem of poor controllability of the mirror support 41 (movable functional portion) in the oscillating motion particularly during the starting and the stopping periods of the movement. The micromirror element X4 as described poses difficulty in precise high-speed driving operation of the mirror support 41.